1. Field of the Invention
The present invention relates to transmitters.
2. Description of the Prior Art
Two wire transmitters have found widespread use in industrial process control systems. A two wire transmitter includes a pair of terminals which are connected in a current loop together with a power source and a load. The two wire transmitter is powered by the loop current flowing through the current loop, and varies the magnitude of the loop current as a function of a parameter P or condition which is sensed. Three and four wire transmitters have separate leads for supply current and output current.
Although a variety of operating ranges are possible, the most widely used two wire transmitter output varies from 4 to 20 mA as a function of the sensed parameter. It is typical with two wire transmitters to provide adjustment of the transmitter so that a minimum or zero value sensed corresponds to the minimum output (for example I.sub.z =4 mA) and that the maximum parameter value to be sensed corresponds to the maximum output (for example 20 mA). This adjustability is typically provided by a zero potentiometer and a span potentiometer which provide variable resistances which can be set by the technician during calibration of the transmitter.
In order to provide a linear relationship between the loop current and the parameter, other adjustments may also be provided. For example, in a two wire transmitter having a variable reactance sensor driven by an oscillator (as shown for example in my previous U.S. Pat. Nos. 3,646,538 and 4,519,253) compensation for nonlinearity can be provided by a variable circuit component or by a component having a specially selected value determined during calibration.
In the case of a pressure sensing transmitter, it is important that the loop current is not affected by changes in temperature of the transmitter. Temperature compensation circuitry is typically provided, and often involves the use of additional resistance adjustments.
The use of resistance adjustments and other circuit components to provide zero, span, linearity and temperature compensation and calibration adds cost to the transmitter, particularly where extremely high resolution circuit components are needed. In addition, the added components themselves introduce potential sources of instability with varying temperature and with shock and vibration of the transmitter.
There is a continuing need for improved transmitters which eliminate the need for separate potentiometers or specially selected components, which provide an easier means for calibrating and, if necessary, recalibrating the transmitter; and which provide greater stability and increased resolution than that normally encountered using potentiometers and the like for calibration.